When Acqualina, a luxury hotel/condo development in Sunny Isles Beach, Fla., opened for sales in 2001, units sold for less than $400 per square foot. Today, those same units are being resold for more than $1,100 per square foot. That’s a nearly 300 percent increase. And it’s great shorthand for the real estate story of this city.

Located in between Bal Harbour and Golden Beach, between the Atlantic Ocean and the Intracoastal, and a 10-mile straight-shot on Collins Avenue/A1A from South Beach, Sunny Isles spent most of the last half of the last century as home to low-rise motels that sat shoulder-to-shoulder right on the beach. Room rates were cheap and watering holes prevalent, both of which drew spring breakers and college kids.

It wasn’t until the 1990s and early 2000s that developers began to see the potential in Sunny Isles. First, the city has proximity to the shopping in tony Bal Harbour and Aventura. Second, developable waterfront land was growing scarcer in South Beach and other parts of nearby Miami, but there was plenty in Sunny Isles to be grabbed. And lastly, land was cheap; those old motels could be picked up for a song.

“We always wanted a beach club, and we knew that people with money would want to live on the ocean,” says Michael Goldstein, president of the Trump Group (no relation to Donald), developer of Acqualina. “The only building that was up when we started Acqualina was Oceana. There were no other towers.”

The ability to build sky-high developments is yet another bonus for Sunny Isles developers. Before he started on the new, 649-foot-high Porsche Design Tower, developer Gil Dezer, president of Dezer Development, worked to get Sunny Isles’ height restrictions up from 550 feet to 649 feet.

“South Beach is difficult to build in because there is no land left. There are only a few sites, and none of them are on the ocean,” says Inigo Ardid, vice president of Key International, which is developing 400 Sunny Isles. “But in Sunny Isles, there is more facing the ocean or right across from the ocean, and you can build taller.”

Today, Sunny Isles boasts one tower after another. And the area is on the cusp of yet another wave of development.

The Acqualina itself is expanding. Under construction next door are the Mansions at Acqualina, 79 4,500-square-foot-plus residences, priced from $1,500 to $3,500 a square foot.

Regalia, with 39 full-floor units starting at 5,500-plus square feet and around $6.5 million, is set to top off in mid-March with move-ins slated for the end of the year. Chateau Beach, 84 condos from 1,500 to 10,000 square feet priced from $1.5 million up to $25 million, launched sales six months ago and began construction in December.

And a handful of other developers are signing contracts and collecting deposits with hopes of starting construction later this year.

At Porsche Design Tower, which will be 132 condos, Dezer is busy converting reservations to signed contracts; 72 units are currently spoken for. At 400 Sunny Isles, a two-tower project on the Intracoastal with 115 condos in each, the first tower is 90 percent under contract, and the project is now taking reservations for the second tower, with construction on both towers slated for this summer.

Jade Signature (the follow-up to Jade Beach and Jade Ocean), a 55-story project with between 210 and 220 units, is still in the design phase. Once plans are finalized, it will pre-sell 60 to 65 percent of the units and then get into the ground, hopefully by the end of this year.

Seems like a lot of new product for a city that’s only 1.4 square miles, water included, but sales are strong. Of the 79 residences at the Mansions at Acqualina, only 11 have yet to go into contract. The sales center opened last March.

“We’ve done over $350 million in sales,” Goldstein says.

What’s most impressive is that these are giant units that are incredibly expensive. The “typical” units are 4,600 square feet and start at $7.75 million. Also in the Mansions building are an 8,500-square-foot unit with an indoor pool on the market for $18 million; an 8,000-square foot penthouse with an indoor pool, which is $25 million; and then the Palazzo d’Oro, a $55 million, two-story, 20,000-square-foot penthouse with a $5 million Fendi Casa furniture package and a glass cantilevered pool that looks straight down 649 feet.

So, who is buying?

“One third of our buyers came from Acqualina to upgrade to the Mansions,” Goldstein says.

The units at Porsche Design Tower are also large and pricey, starting at 4,200 square feet and $4 million. There are two 15,000-square-foot penthouses.

All of which is doubly ambitious when you know that developers in Sunny Isles are asking anywhere from a 30 to 60 percent down payment over the course of sales and construction.

“It’s all pre-sales,” Ardid says. “The difference is that now the amount of the deposit is a lot larger.”

At 400 Sunny Isles, where units in the second tower are 1,500 to 4,000 square feet, and $750,000 to $3.5 million, Key International is asking for 50 percent down.

Regalia had planned on requiring 60 percent: 20 percent on signing the contract, 20 percent when the buyer’s floor was poured and 20 percent when the building was topped out. But because it has gone ahead with construction rather than waiting for 65 percent of the building to sell, it is now asking 20 percent at signing, another 20 percent after 90 days and another 20 percent 90 days after that. The units in the building are nearly 50 percent sold.

This is a long way of saying that Sunny Isles buyers are people, many of them international, with a lot of cash.

“Our penthouse buyer came out of the blue. This guy’s so well-known that if I gave you the country, you’d know who it is, or someone in the financial world would know,” says Dezer, referring to Porsche Design Tower, which will have a car elevator that recognizes the owner by his automobile and takes him straight to his apartment and assigned parking space. “We have a famous race-car driver, athletes, actors; they can all come and go from their apartment without seeing anyone.”